Use of aquifers for groundwater energy storage for heating and cooling buildings in urban settings is common. Environmental engineers and policy makers are recognizing the “positive side-effects” of such open-loop extraction and injection based heating-cooling systems for contaminant attenuation in urban groundwater legacy plumes (European Environment Agency, 2007. State of the Environment. EEA, Copenhagen). Such contaminant attenuation is presumably attributed to enhanced mixing of nutrients and microorganisms in the subsurface (Slenders, H. L. A., Dols, P., Verburg, R., de Vries, A. J (2010), Sustainable synergies for the subsurface: Combining Groundwater Energy with Remediation. Remediation, Spring 2010 (D01:10.1002; Wiley Periodicals, Inc.); Verburg, R., Slenders, H. L. A., Hoekstra, N., Van Nieuwkerk, E., Guijt, R., Van der Mark, B., (2010). Manual BOEG: Underground Thermal Energy and Groundwater Contamination (in Dutch)). These geothermal systems typically operate at low temperatures (<25° C.) and rely heavily on groundwater recirculation (extraction and injection).
In addition, the effects of such open-loop systems on chlorinated hydrocarbon plumes were evaluated by K. G. Zuurbier et al in 2013 where they noted several “negative side-effects” including: limited temperature increase (<15° C.) to stimulate significant thermally-enhanced biodegradation; and a potential for increasing the contaminant plume size (Zuurbier, K. G., Hartog, N., Valster, J., Post, V., Breukelen, B. (2013). The impact of low temperature seasonal aquifer thermal energy storage (SATES) systems on chlorinated solvent contaminated groundwater: Modelling of spreading and degradation). Additionally, such open-loop groundwater pumping based geothermal systems are only feasible in an urban setting where a large number of geothermal-energy-equipped dwellings already exist.
Effects of temperature on contaminant degradation are well documented from various scientific perspectives. Comparing degradation of petroleum hydrocarbons in soils at 10° C. and 20° C., Margesin et al. reported three times faster degradation of benzene, toluene, ethylbenzene and xylene at higher temperature (Margesin, R., Schinner, F. (2001), Biodegradation and bioremediation of hydrocarbons in extreme environments. Applied Microbial Biotechnology Vol. 56 p. 650-663). Studies on biodegradation rates due to increase in temperature (5-50° C.) have shown peak degradation rates at 30 and 40° C. for petroleum hydrocarbons (Xu, J. G. (1997), Biodegradation of Petroleum Hydrocarbons in Soil as Affected by Heating and Forced. Aeration. Journal of Environmental Quality, Vol. 26 No. 6, p. 1511-1516; See also MS thesis—Thermally enhanced bioremediation of LNAPL by Zeman, Natalie Rae, M.S., COLORADO STATE UNIVERSITY, 2013, 139 pages). Additionally, some chlorinated volatile organic compounds, specifically chlorinated alkanes such as 1,1,1-trichloroethane, 1,2-dichloroethane, and carbon tetrachloride, readily undergo hydrolysis reactions under elevated temperatures which can lead to their in-situ destruction (Suthersan, S., Horst, J., Klemmer, M., Malone, D. (2012), Temperature-Activated Auto-Decomposition Reactions: An Underutilized In Situ Remediation Solution. Ground Water Monitoring & Remediation Vol. 32 No. 3 p. 34-40). Both biodegradation and hydrolysis rates, are temperature dependent, and approximately double for each 10° C. increase in temperature.
Successful implementation of ex-situ treatment of mercury contaminated soil by use of a solar reflection approach was reported by Navarro (Application of solar thermal desorption to remediation of mercury-contaminated soils. A. Navarro, I. Canadas, D. Martinez, J. Rodriguez, J. L. Mendoza. Department of Fluid Mechanics, Polytechnic University of Catalonia (UPC), ETSEIT, Colo. n 11, 08222 Terrassa, Barcelona, Spain, Plataforma Solar de Almeria (PSA), Solar Platform of Almeria-CIEMAT, P.O. Box 22, Tabernas, E-04200 Almeria, Spain.)
Physical extraction systems (ex-situ treatment of extracted groundwater or application of air-sparge-soil-vapor extraction) to remediate contamination associated with non-aqueous phase liquids (NAPLs) are a common, traditional and relatively expensive remediation approach. In addition to the capital costs, operation and maintenance costs could be a limiting factor for full scale implementation of such technologies. With increase in groundwater temperature, solubility of NAPLs increases and viscosity decreases (Thermal variation of organic fluid properties and impact on thermal remediation feasibility, DOI: 10.1080/15320389709383566 Brent E. Sleepa & Yanfang Maa pages 281-306) both of these properties would enhance the recovery of contaminant mass in the dissolved as well as vapor phase, thus significantly improving the treatment efficiency of a physical extraction system. Subsequently, remediation time and life cycle costs of a physical extraction system can be significantly reduced if implemented in conjunction with TISR.
Soil, like all other materials, has thermal properties that allow heat to propagate through the subsurface. Heat transport can occur through two processes: conduction through soil solids (soil particles) and advection through ambient groundwater flow. Heat transfer in soils depends on soil thermal conductivity and heat capacity; these thermal properties are a function of the soil mineral composition, soil density, porosity and saturation with water or air and can be determined by using well established field and laboratory methods or estimated from the existing scientific literature.
Solar thermal collection is a common technique used to collect heat by capturing solar radiation. Globally, solar radiation can vary based on the distance from the equator as well seasonal and/or daily weather changes. Additionally, inherent geothermal heat storage allows for buffering low heat input days caused by low solar radiation on cloudy days and/or due to seasonal changes.
Contaminant degradation and associated remediation is a function of the temperature for both biological and abiotic processes occurring in soils and groundwater. As the subsurface temperature rises, reaction rates increase concurrent with an increased rate of contaminant degradation.